Assign unused Future return value to a variable

Error Prone plans to make this pattern into a compile error; this avoids
future breakage.

In some of these cases, it's not obvious that ignoring the return value
is the correct thing to do; the hope is that the construct is ugly
enough to make the author think twice. In many cases though, it's clear
that a locally-created Runnable or Callable already does its own error
handling, so we can leave a comment alongside the ignored return value.

Eventually many of these should be audited again, and we can replace
some of the @SuppressWarnings with Error Prone's @CanIgnoreReturnValue.
That much is left for future cleanup.

Change-Id: Ia989214d85e0d6c387e388a77178e0b5c4bf2498

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/GerritServer.java

@@ -49,6 +49,7 @@ import java.util.concurrent.Callable;
 import java.util.concurrent.CyclicBarrier;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import org.apache.log4j.Level;
 import org.apache.log4j.Logger;
@@ -169,6 +170,8 @@ public class GerritServer {
     } else {
       site = initSite(cfg);
       daemonService = Executors.newSingleThreadExecutor();
+      @SuppressWarnings("unused")
+      Future<?> possiblyIgnoredError =
           daemonService.submit(
               new Callable<Void>() {
                 @Override
@@ -176,7 +179,11 @@ public class GerritServer {
                   int rc =
                       daemon.main(
                           new String[] {
-                        "-d", site.getPath(), "--headless", "--console-log", "--show-stack-trace",
+                            "-d",
+                            site.getPath(),
+                            "--headless",
+                            "--console-log",
+                            "--show-stack-trace",
                           });
                   if (rc != 0) {
                     System.err.println("Failed to start Gerrit daemon");

gerrit-cache-h2/src/main/java/com/google/gerrit/server/cache/h2/H2CacheFactory.java

@@ -39,6 +39,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import org.eclipse.jgit.lib.Config;
@@ -116,6 +117,8 @@ class H2CacheFactory implements PersistentCacheFactory, LifecycleListener {
     if (executor != null) {
       for (final H2CacheImpl<?, ?> cache : caches) {
         executor.execute(cache::start);
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError =
             cleanup.schedule(() -> cache.prune(cleanup), 30, TimeUnit.SECONDS);
       }
     }

gerrit-cache-h2/src/main/java/com/google/gerrit/server/cache/h2/H2CacheImpl.java

@@ -46,6 +46,7 @@ import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executor;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
@@ -196,6 +197,8 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> implements Per
     cal.add(Calendar.DAY_OF_MONTH, 1);
 
     long delay = cal.getTimeInMillis() - TimeUtil.nowMs();
+    @SuppressWarnings("unused")
+    Future<?> possiblyIgnoredError =
         service.schedule(() -> prune(service), delay, TimeUnit.MILLISECONDS);
   }
 

gerrit-lucene/src/main/java/com/google/gerrit/lucene/AbstractLuceneIndex.java

@@ -40,6 +40,7 @@ import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executor;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
@@ -116,6 +117,8 @@ public abstract class AbstractLuceneIndex<K, V> implements Index<K, V> {
                   .setNameFormat(index + " Commit-%d")
                   .setDaemon(true)
                   .build());
+      @SuppressWarnings("unused") // Error handling within Runnable.
+      Future<?> possiblyIgnoredError =
           autoCommitExecutor.scheduleAtFixedRate(
               new Runnable() {
                 @Override
@@ -135,7 +138,8 @@ public abstract class AbstractLuceneIndex<K, V> implements Index<K, V> {
                       log.error(
                           "SEVERE: Error closing "
                               + index
-                          + " Lucene index  after OOM; index may be corrupted.",
+                              + " Lucene index after OOM;"
+                              + " index may be corrupted.",
                           e);
                     }
                   }

gerrit-pgm/src/main/java/com/google/gerrit/pgm/util/LogFileCompressor.java

@@ -31,6 +31,7 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.time.LocalDate;
 import java.time.ZoneId;
+import java.util.concurrent.Future;
 import java.util.zip.GZIPOutputStream;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -63,9 +64,12 @@ public class LogFileCompressor implements Runnable {
       ZoneId zone = ZoneId.systemDefault();
       LocalDate now = LocalDate.now(zone);
       long milliSecondsUntil11pm = now.atStartOfDay(zone).plusHours(23).toInstant().toEpochMilli();
+      @SuppressWarnings("unused")
+      Future<?> possiblyIgnoredError =
           queue
               .getDefaultQueue()
-          .scheduleAtFixedRate(compressor, milliSecondsUntil11pm, HOURS.toMillis(24), MILLISECONDS);
+              .scheduleAtFixedRate(
+                  compressor, milliSecondsUntil11pm, HOURS.toMillis(24), MILLISECONDS);
     }
 
     @Override

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeCleanupRunner.java

@@ -25,6 +25,7 @@ import com.google.gerrit.server.util.ManualRequestContext;
 import com.google.gerrit.server.util.OneOffRequestContext;
 import com.google.gwtorm.server.OrmException;
 import com.google.inject.Inject;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -64,7 +65,11 @@ public class ChangeCleanupRunner implements Runnable {
             String.format(
                 "Ignoring invalid changeCleanup schedule configuration: %s", scheduleConfig));
       } else {
-        queue.getDefaultQueue().scheduleAtFixedRate(runner, delay, interval, TimeUnit.MILLISECONDS);
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError =
+            queue
+                .getDefaultQueue()
+                .scheduleAtFixedRate(runner, delay, interval, TimeUnit.MILLISECONDS);
       }
     }
 

gerrit-server/src/main/java/com/google/gerrit/server/change/ChangeInserter.java

@@ -70,6 +70,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.transport.ReceiveCommand;
@@ -449,6 +450,8 @@ public class ChangeInserter extends BatchUpdate.InsertChangeOp {
             }
           };
       if (requestScopePropagator != null) {
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError =
             sendEmailExecutor.submit(requestScopePropagator.wrap(sender));
       } else {
         sender.run();

gerrit-server/src/main/java/com/google/gerrit/server/change/EmailReviewComments.java

@@ -42,6 +42,7 @@ import com.google.inject.ProvisionException;
 import com.google.inject.assistedinject.Assisted;
 import java.util.List;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -111,7 +112,8 @@ public class EmailReviewComments implements Runnable, RequestContext {
   }
 
   void sendAsync() {
-    sendEmailsExecutor.submit(this);
+    @SuppressWarnings("unused")
+    Future<?> possiblyIgnoredError = sendEmailsExecutor.submit(this);
   }
 
   @Override

gerrit-server/src/main/java/com/google/gerrit/server/git/EmailMerge.java

@@ -35,6 +35,7 @@ import com.google.inject.Provider;
 import com.google.inject.ProvisionException;
 import com.google.inject.assistedinject.Assisted;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -89,7 +90,8 @@ public class EmailMerge implements Runnable, RequestContext {
   }
 
   public void sendAsync() {
-    sendEmailsExecutor.submit(this);
+    @SuppressWarnings("unused")
+    Future<?> possiblyIgnoredError = sendEmailsExecutor.submit(this);
   }
 
   @Override

gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollectionRunner.java

@@ -22,6 +22,7 @@ import com.google.gerrit.server.config.GcConfig;
 import com.google.gerrit.server.config.ScheduleConfig;
 import com.google.gerrit.server.project.ProjectCache;
 import com.google.inject.Inject;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -52,6 +53,8 @@ public class GarbageCollectionRunner implements Runnable {
       } else if (delay < 0 || interval <= 0) {
         gcLog.warn(String.format("Ignoring invalid gc schedule configuration: %s", scheduleConfig));
       } else {
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError =
             queue
                 .getDefaultQueue()
                 .scheduleAtFixedRate(gcRunner, delay, interval, TimeUnit.MILLISECONDS);

gerrit-server/src/main/java/com/google/gerrit/server/git/MergedByPushOp.java

@@ -39,6 +39,7 @@ import com.google.inject.assistedinject.AssistedInject;
 import java.io.IOException;
 import java.util.Collections;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Repository;
@@ -164,6 +165,8 @@ public class MergedByPushOp extends BatchUpdate.Op {
     if (!correctBranch) {
       return;
     }
+    @SuppressWarnings("unused") // Runnable already handles errors
+    Future<?> possiblyIgnoredError =
         sendEmailExecutor.submit(
             requestScopePropagator.wrap(
                 new Runnable() {

gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java

@@ -138,6 +138,7 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
@@ -1959,8 +1960,9 @@ public class ReceiveCommits {
       Change change = notes.getChange();
       if (change.getDest().equals(magicBranch.dest)) {
         logDebug("Found change {} from existing refs.", change.getKey());
-        // reindex the change asynchronously
-        indexer.indexAsync(project.getNameKey(), change.getId());
+        // Reindex the change asynchronously, ignoring errors.
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError = indexer.indexAsync(project.getNameKey(), change.getId());
         return true;
       }
     }

gerrit-server/src/main/java/com/google/gerrit/server/git/RenameGroupOp.java

@@ -23,6 +23,7 @@ import com.google.inject.assistedinject.Assisted;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.errors.RepositoryNotFoundException;
@@ -92,11 +93,12 @@ public class RenameGroupOp extends DefaultQueueOp {
       }
     }
 
-    // If one or more projects did not update, wait 5 minutes
-    // and give it another attempt.
+    // If one or more projects did not update, wait 5 minutes and give it
+    // another attempt. If it doesn't update after that, give up.
     if (!retryOn.isEmpty() && !tryingAgain) {
       tryingAgain = true;
-      start(5, TimeUnit.MINUTES);
+      @SuppressWarnings("unused")
+      Future<?> possiblyIgnoredError = start(5, TimeUnit.MINUTES);
     }
   }
 

gerrit-server/src/main/java/com/google/gerrit/server/git/ReplaceOp.java

@@ -61,6 +61,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Future;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Ref;
@@ -412,6 +413,8 @@ public class ReplaceOp extends BatchUpdate.Op {
           };
 
       if (requestScopePropagator != null) {
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError =
             sendEmailExecutor.submit(requestScopePropagator.wrap(sender));
       } else {
         sender.run();

gerrit-server/src/main/java/com/google/gerrit/server/group/PutName.java

@@ -39,6 +39,7 @@ import java.io.IOException;
 import java.util.Collections;
 import java.util.Date;
 import java.util.TimeZone;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 
 @Singleton
@@ -125,6 +126,8 @@ public class PutName implements RestModifyView<GroupResource, Input> {
 
     groupCache.evict(group);
     groupCache.evictAfterRename(old, key);
+    @SuppressWarnings("unused")
+    Future<?> possiblyIgnoredError =
         renameGroupOpFactory
             .create(
                 currentUser.get().newCommitterIdent(new Date(), TimeZone.getDefault()),

gerrit-server/src/main/java/com/google/gerrit/server/index/change/ChangeIndexer.java

@@ -52,6 +52,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.Callable;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
 import java.util.concurrent.atomic.AtomicReference;
 import org.eclipse.jgit.lib.Config;
 import org.slf4j.Logger;
@@ -313,7 +314,9 @@ public class ChangeIndexer {
 
   private void reindexAfterIndexUpdate(Project.NameKey project, Change.Id id) {
     if (reindexAfterIndexUpdate) {
-      reindexIfStale(project, id);
+      // Don't retry indefinitely; if this fails the change will be stale.
+      @SuppressWarnings("unused")
+      Future<?> possiblyIgnoredError = reindexIfStale(project, id);
     }
   }
 

gerrit-server/src/main/java/com/google/gerrit/server/index/change/ReindexAfterUpdate.java

@@ -39,6 +39,7 @@ import com.google.inject.Provider;
 import java.io.IOException;
 import java.util.List;
 import java.util.concurrent.Callable;
+import java.util.concurrent.Future;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -81,7 +82,9 @@ public class ReindexAfterUpdate implements GitReferenceUpdatedListener {
           @Override
           public void onSuccess(List<Change> changes) {
             for (Change c : changes) {
-              executor.submit(new Index(event, c.getId()));
+              // Don't retry indefinitely; if this fails changes may be stale.
+              @SuppressWarnings("unused")
+              Future<?> possiblyIgnoredError = executor.submit(new Index(event, c.getId()));
             }
           }
 

gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/MailReceiver.java

@@ -28,6 +28,7 @@ import java.util.Set;
 import java.util.Timer;
 import java.util.TimerTask;
 import java.util.concurrent.Callable;
+import java.util.concurrent.Future;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -133,7 +134,8 @@ public abstract class MailReceiver implements LifecycleListener {
               }
               return null;
             };
-        workQueue.getDefaultQueue().submit(task);
+        @SuppressWarnings("unused")
+        Future<?> possiblyIgnoredError = workQueue.getDefaultQueue().submit(task);
       } else {
         // Synchronous processing is used only in tests.
         try {

gerrit-server/src/main/java/com/google/gerrit/server/project/ProjectCacheClock.java

@@ -20,6 +20,7 @@ import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import org.eclipse.jgit.lib.Config;
@@ -50,14 +51,13 @@ public class ProjectCacheClock {
                   .setDaemon(true)
                   .setPriority(Thread.MIN_PRIORITY)
                   .build());
+      @SuppressWarnings("unused") // Runnable already handles errors
+      Future<?> possiblyIgnoredError =
           executor.scheduleAtFixedRate(
-          new Runnable() {
-            @Override
-            public void run() {
-              // This is not exactly thread-safe, but is OK for our use.
-              // The only thread that writes the volatile is this task.
+              () -> {
+                // This is not exactly thread-safe, but is OK for our use. The only
+                // thread that writes the volatile is this task.
                 generation = generation + 1;
-            }
               },
               checkFrequencyMillis,
               checkFrequencyMillis,